1. Field of the Invention
The present invention relates to a Raman amplifier for wideband Raman amplification of wavelength division multiplexed (WDM) signal lights.
2. Description of the Related Art
In a wavelength division multiplexing (WDM) optical communication system, a plurality of signal lights at different wavelengths are multiplexed together, and then transmitted through a single optical fiber operating as an optical transmission line. Such a WDM optical communication system has a very high capacity for information transmission, and is currently being used as a basic technology for backbone networks supporting Internet services.
To transmit signal lights over long distances, optical amplifiers are positioned along the transmission line, to amplify the multiplexed signal lights. Such optical amplifiers include rare-earth element dope optical fiber amplifiers, such as erbium doped optical fiber amplifiers (EDFA), and Raman amplifiers.
In a WDM optical communication system using an optical amplifier, the maximum limit of optical transmission power is restricted by the degradation of light waveform and cross-talk caused by nonlinear optical effects in the transmission line.
Moreover, the minimum limit of optical transmission power is restricted by the deterioration of optical signal-to-noise ratio (SNR) caused by optical noise, e.g. amplitude stimulated emission light, generated by the optical amplifier.
For a high-capacity WDM optical communication system which multiplexes tens to hundreds of signal lights together, the use of Raman amplifiers together with EDFAs installed at intervals of several tens of kilometers is regarded as a promising technology to compensate for loss in the transmission line by reducing nonlinear optical effects and deterioration of optical SNR.
Various combinations of Raman amplifiers with EDFAs have been proposed, but such proposals do not consider the situation where the number of multiplexed signal lights at different wavelengths is further increased to thereby increase capacity of the system.